TL;DR As soon as your colleague accepts the increase, or if your colleague is already on a higher rate plan, your monthly contribution will also increase.

It is important to know that the rate plan of a colleague’s subscription cannot be lower than the rate plan proposed by your company.

There are three possibilities:

your colleague was on the “old” rate plan (e.g. €15): he/she is given the option to activate the newly proposed rate plan (€20), or a higher plan rate (€29). In the latter case, he/she takes care of the monthly price difference.
your colleague was on a higher rate plan (€20) than that initially proposed by the company (€15), but on a lower rate plan than the newly proposed rate plan (€29): he/she will be on the new rate plan (€29), and no price difference will be billed to him/her.
your colleague was on a higher rate plan (€29) than that initially proposed by the company (€15), and also on a higher rate plan than the newly proposed rate plan (€20): he/she remains on the new rate plan (€29), and he/she will be billed a smaller price difference.

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