Firstly it's Stansted not Stanstard.

If you are going Milton Keynes to Stansted, you can do it mostly above ground or upper level by taking:

- Avanti train to Euston
- walk to Euston Square
- H&C/Circle Line to Liverpool Street
- Stansted Express Liverpool Street to Stansted

or

- Avanti to Euston
- Victoria Line to Tottenham Hale
- Stansted Express Tottenham Hale to Stansted

But if you think you will get a solid reliable data connection on *any* of those train legs, then you are very much mistaken.

The sensible thing is to get to Stansted before 11 and find somewhere quiet, maybe a hotel, to do the call from there.