Use a virtual telephone number via VoIP with your mobile phone

If you want to be reachable via a landline number while on the move, a virtual telephone number is a perfect solution for you. There are different ways you can with your phone via the local number to reach are.

Forwarding to a mobile phone number. You can store your mobile phone number as a forwarding destination in the telephone system.

You can then link this to the virtual landline phone number so that all calls that go to your telephone number are directed to your cell phone number. You can of course also set this time-based. If you are reachable in the office during office hours, for example, you can set your office telephone as the destination for this time.

If you would also like to be reachable beyond this, simply forward the number to your cell phone number outside of office hours. Important. Forwarding to a cell phone number is a new call that is established via the telephone system.

As a result, the usual connection charges apply. Forwarding to a cell phone number costs 19 cents (22.61 cents incl. VAT) per minute in the FREE tariff and 10 cents (11.9 cents incl. VAT) per minute in the PLUS tariff. Forwarding is not covered by any flat rates.

Forwarding to a SIP call app. In the app stores of your smartphone, you will find a number of SIP call apps with which you can implement VoIP telephony on your mobile phone.

Many are available free of charge or in a freemium version – i.e. a basic version that enables telephony and a premium paid version that brings additional functionalities.

Other apps are subject to a fee. Which app you choose depends on your personal preferences. As a rule, all of them can be used with the telephone system. You can find an overview of the apps we have tested in our online help under ” Mobile Integration “.

With an app, you then have a fully-fledged IP terminal that you can, of course, also link to a phone number based on time. You can then make and receive calls using your virtual telephone mobile number – at a landline tariff or, if you have booked a flat rate, covered by your flat rate.

To use a SIP call app, you only need an internet connection. It does not matter whether you are logged in to the WLAN or connected via your mobile data package. A good and stable connection with enough bandwidth is important.

Up to 100 kBit / s occur upstream and downstream per call – this should be possible for your internet connection in order to be able to make calls without interruption and in good quality.

Of course, it doesn’t matter where you are. Whether you make a phone call in your office using your stationery office phone, answer the call via your smartphone via your home WiFi network, or receive a call using your data packet on your landline number during your summer vacation. it makes no difference to the caller where you are. Even more. the caller wo n’t even notice.

A virtual telephone number is a perfect companion for freelancers, home office employees, business travelers, field staff, digital nomads, or even for users who are just on the road a lot and want to be reachable via a correct landline number.

VoIP with the smartphone

In addition to their conventional office telephone system, many companies also have separate mobile phone contracts for their employees so that they can also be reached on the go.

Mobile accessibility is essential, especially if you employ a large number of employees in the field or if you often have to make external appointments.

With the call-through function, you can also make calls on the go with your virtual telephone number via your online telephone system and still keep your office telephone number.

Free SIP call apps for mobile VoIP telephony

For many, one of the main advantages of a free mobile number system from the cloud is the ability to use it anywhere. Everyone wants and needs to be flexible and mobile in today’s professional life, but of course still accessible.

However, so that colleagues, customers, and Co. do not have to be given several phone numbers where, admittedly, no one can know which one you can be reached at, it makes sense to always be reachable under the traditional office number.

Of course, this can be done simply by forwarding your office number to your cell phone number – but of course, it costs. It is better and above all cheaper, with a SIP call, app to make calls directly via the telephone system.

This means that you can not only be reached on the go via your landline number, you can also use it to make calls – depending on the landline tariff or even covered by the booked flat rate. This is possible with a cloud telephone system.

Now enter “SIP App”, “Call App” or “SIP Call App” in the app store of your smartphone – the results are varied and quantitatively almost overwhelming.

Often, the classic SIP call apps, in which you can enter the SIP user data of your VoIP provider and use your service to make calls, also mix with call apps that do not know SIP or only with other devices on which the corresponding app is installed is able to communicate.

o that you can find the right one for your purposes, we have once again taken a look at and compared the common apps.

What is Callthrough?

Callthrough is generally a procedure that mediates telephone calls through a provider that is not the operator of the connection used.

For example, you can use a landline number on your cell phone and thus reduce call costs. Callthrough generally works as follows.

The caller dials in with the dial-in number of the call-through provider and verifies himself by entering a PIN or voice control. Various apps for smartphones automate this process so that only the entry of the target number is required.

You can also use callthrough with your smartphone via VoIP and your virtual telephone system (if the provider offers the function and you have activated it). Mobile apps or certain settings in your device can implement call-through.

This means that you can use the tariffs and functions of your virtual telephone system while on the move and can be reached anywhere with a phone number.

Your mobile phone connects via an available WLAN or the mobile data network. However, this also means that high costs for the mobile data can be incurred or a bad data connection can hamper the call.

How Callthrough Works

To prevent this, some providers of virtual telephone systems offer a certain form of the call through. The application dials into your provider via the GSM network and connects you to the other party using your landline phone number.

In this way, you conserve your data volume and avoid the problem of poor data connections. Many providers of virtual telephone systems offer smartphone apps with a call-through function via the GSM network.

To do this, you usually have to store the same data that is available on your stationary IP telephone. The app also needs your mobile phone number in order to be able to assign it to the account.

Here you should note that you can only use your mobile phone number once, otherwise the service will not work because the assignment fails.

Some Android smartphones support the IP function without having to install their own app. To do this, you have to search for the “Profiles” menu item in the settings and create a new SIP profile there.

Enter the phone number you received from your VoIP provider, your password, and the server of your provider. To activate Internet telephony on your Android smartphone, you must set up the SIP profile as the primary account. Know about, Best apps to call through VoIP Telephony.


There is certainly the best apps for mobile VoIP for every user. They are sometimes the best way to be reached via the landline number and to be able to make calls at a landline rate using a cell phone.

However, as a user, you should pay attention to one thing. To be reachable, you need a stable internet connection. This can be lacking in rural areas. Either way, SIP Call Apps offer the greatest possible flexibility with good usability – and the market for it is now so large that everyone can find the right one for them.