The
contact page on MEN's website says that the 0871 number you give is for General enquiries and iscynically charged at 10p/min at all times from a BT landline; that's more expensive than 0870 and no pricing information is given by MEN Arena!
There are number in the database for Ticketmaster. Just search for err "Ticketmaster".
As for contacting the MEN Arena and not being charged the earth, 0161 9505000 is in the database with a note: "This is the number for customers with special needs requirements". I've just called it and it answers with an automated menu; it possibly could be the same destination as the 0871, but I'm not calling the 0871 to find out. If you select the option to book tickets, you are given the 0870 number.
On the MEN Arena website it gives 0161 950 5229 as the special needs, so numbers probably are 0161 9505xxx.