George Phil Apr 29, 2015:

Actually, as you are talking about “headed notepaper”, it makes difference and depends on particular company and its bussiness.

When, for any reason, a company puts a mobile/cell number on a formal notepaper, it is common practice to indicate it as suggested by the peers above (just to avoid misleading).

There are two concerns behind indicating mobile as a “phone number”. It may suggest a desire to accept the incoming calls on 24/7 basis, and does not necessarily applies to support services. It might be a part of a marketing strategy (as some domestic SMB do).
On the other hand, it may be a sign that the company does not have a permanent office at all.

So, you should be cautious about that)).

Evgeny Artemov (X) Apr 29, 2015:

State Моб. if you have a landline number too. Not really necessary if the mobile is alone. Its usually easy to guess it by the "9" in the prefix after "+7" or "8".