I can only imagine it means 'switchboard' or something like that, i.e. several people can ring the number at the same time and still get through.

"79 à 89, rue Henri - Gauthier 93000 BOBIGNY Tél. : (1) 48.44.81.50 (JONCTIONS MULTIPLES)"

"Seul service agréé par la Fédération Mutualiste de la Région Parisienne Bureaux Ouvert aux Mutualistes et à tous les assurés sociaux 01.43.29.07.50. (JONCTIONS MULTIPLES) 24h/24"

"Tel. 04 50 22 20 38 (JONCTIONS MULTIPLES) Telecopie: 0450222904"


There's a lot on the Web about MULTI-LINE PHONE SYSTEMS:
"What is a multi-line phone system? A multi-line phone system lets you handle two or more calls simultaneously. It enables employees to place calls on hold, dial internal or external numbers, and return to the line. Multi-line phones differ from phones with a single line that can only maintain one phone call at a time"



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Note added at 1 hr (2022-04-04 09:01:18 GMT)
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That said, I don't see the point in translating it or even putting it in French to start with! It changes nothing for the user. It simply suggests that the phone number is that of some tin-pot organization that is bragging about the fact that it has more than a simple domestic phone line. I can't say I can recall having ever seen anything like it in English, not even for small firms.