You will either have to return to Pakistan and marry there, or apply for a fiance visa and marry in the UK. Here's the UKVI official ruling on telephone marriages:
3.2. Telephone marriages
The formal validity of a telephone marriage is to be determined according to the laws of the countries in which both parties are physically present when the marriage takes place. Therefore, a telephone marriage celebrated whilst one of the parties is in the United Kingdom will not be valid as telephone marriages are not valid in this country.
In cases where the UK-based sponsor was overseas when a telephone marriage took place and the laws of both countries recognise such marriages, we cannot deny that the marriage is valid. Enquiries about the marriage laws of other countries may be referred to Policy.
(Casa, not CR001)
Please don't send me PMs asking for immigration advice on posts that are on the open forum. If I haven't responded there, it's because I don't have the answer. I'm a moderator, not a legal professional.