There’s no single ISO that will reliably work for unknown film. The practical approach is to test one roll first.

A good starting point is EI/ISO 200, since it’s a reasonable middle speed. Shoot a variety of scenes and bracket exposures across several frames, keeping notes of the settings. Including a few scenes metered with your phone can give you known reference exposures.

Then develop that roll and see which frames look best. If the whole roll looks underexposed or overexposed, adjust the EI for the remaining rolls accordingly. This only works well if all the rolls are actually the same film, so check whether they look identical.

If possible, ask the seller for any information. Also inspect the film for edge markings, which may reveal the film type or speed. If you want to avoid wasting a whole roll, a lab may be able to do a clip test by developing just the first few exposed frames.

In short: start around ISO 200, test carefully, review results, and adjust from there.