Speaking in the premises of the Masjid is against the etiquette and the sanctity of the Masjid, and has been classified as Makrooh (disliked) by the Jurists. This ruling will be with regard to vain and baseless speech and worldly conversation, as mentioned by the Jurists. However, based on the nature of the speech the ruling will differ. 

If one engages in essential and necessary speech in the Masjid, this will be deemed as permissible. Although, even in such a case it would be preferable to leave the Masjid premises for such speech. On the other hand, if the nature of the discussion is of Haram i.e. discussions which contain vulgarity and immorality etc., then to engage in such discussions in the Masjid would, without a doubt, only amplify the severity of the crime. 

The Jurists have also mentioned that to inhabit the Masjid solely for the purpose of worldly conversation has also been prohibited. However, concerning your situation in query, we do not see a necessity to speak on the phone in the premises of the Masjid while out in Tabligh Jamaat, as one is not confined to the Masjid boundary. 

Once again, if it is possible to leave the Masjid for such discussions (as is the probable scenario) one should do so. One should also make the Niyyah (intention) of I’tikaaf before engaging in such actions in the Masjid, as is advised in Tabligh Jamaat. One should also observe caution in ensuring that no individual is disturbed in his worship by his speech. Generally speaking, to engage in wordly conversation in the Masjid (without a valid reason and necessity) is a sinful act and those who engage in it should be reprimanded by those of authority.